Bartels’ rotation number

The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartels to begin in January 1833, and the count has continued by 27-day intervals to the present.  The Sun has an average rotation period (as seen from the Earth) of 27.27 days.  Therefore, solar longitude slowly drifts with respect to the Bartels rate. Compare Carrington longitude.